1. Field of the Invention
The present invention relates generally to valve actuators and, more particularly, to apparatus and methods for an actuator seal bearing assembly.
2. Description of the Background
Pneumatic actuators for pneumatically controlling valves are used throughout industry for a wide variety of purposes. For some applications, it is highly desirable that the actuator be as compact as possible because physical space for the valve and actuator is quite limited.
An exemplary and highly compact configuration for a pneumatic actuator is shown in U.S. Pat. No. 4,354,424, issued Oct. 19, 1982, to Sven Nordlund, which is hereby incorporated herein by reference. In that actuator, each actuator piston is provided with a rack having teeth thereon to engage corresponding teeth of an operating element. The rack includes a recess for receiving a first spring that produces a return force on the piston. The recess extends substantially into the rack of the piston. A second shorter spring is provided within a central portion of each actuator piston. Thus, one spring in each piston is off-set from the center and is longer than the other spring. The offset, longer spring provides a solution to a basic problem of compact spring return actuators. Prior to this invention, such actuators did not consistently have sufficient spring force to completely close the valve. While the off-set spring is therefore very advantageous, when the actuator is operated under full or near full load over a large number of cycles, a very slight tilting of the actuator piston can occur that may cause scoring of the cylinder and reduce the lifetime operation of the actuator. While most control systems are designed with ample safety factors that place the actuator under a relatively light load, a particular system may be designed to require a higher load with a small actuator due to limited space. As well, a system component of any control system may change or malfunction and thereby place a higher load on the actuator until the problem is detected and then corrected. Thus, it would be very desirable to increase the reliability of the compact actuator for the circumstance of high loading with continuous cycling operation.
While it would be desirable to limit wear even under a maximum load scenario, there are constraints. It would be desirable to eliminate the wear without increasing the length of the piston. The extremely compact configuration of the exemplary actuator discussed above is another significant advantage of this design. Moreover, it would be desirable to provide a means to easily update existing actuators so that if and when they are operated at full or near full load, then the lifetime will be substantially as extensive as though it were operated at only a much smaller load. Those skilled in the art have long sought and will appreciate the present invention which addresses these and other problems.